This invention relates to a pneumatic servobrake and to its assembling process. The object of the invention consists in simplifying the manufacture of such servobrakes for a better cost control and, in the first place, for a more reliable structure.
Fundamentally, a pneumatic servobrake comprises a variable-volume front chamber, separated from a variable-volume rear chamber by a partition wall, consisting of a tight flexible diaphragm, and by a rigid skirt-forming plate. The rigid skirt drives a pneumatic piston which bears, through a push rod, on the primary piston of a master cylinder of a hydraulic braking system, typically a tandem master cylinder. The front chamber, directed towards the master cylinder, is pneumatically connected to a vacuum source whereas the rear chamber, in the opposite direction to the front chamber and on the side of a brake pedal, is pneumatically connected, under a valve control, to a propellant fluid source, typically atmospheric-pressure air. At rest, that is when the driver does not depress the brake pedal, the front and rear chambers are interconnected, while the rear chamber is isolated from the atmospheric pressure. On braking, first the front chamber is isolated from the rear chamber and then air is admitted into the rear chamber. Such air entrance imparts a propulsive motion to the partition wall, thus actuating the pneumatic braking assistance.
Besides, hydraulic brake assisting systems are known, in which an electric motor is conventionally connected to a hydraulic pump, injecting a pressure fluid into the brake circuit when it is actuated. The electric motor is controlled on the basis of the measurement of the pressures within the front and rear chambers of the pneumatic servobrake. Therefore, two pressure sensors are pneumatically connected to each chamber, so as to measure the fluid pressure in it. These sensors output electrical signals, indicative of the pressures. For convenience sake, the pressure sensors are installed on an outer wall of the front chamber, close to the master cylinder. As regards the measurement of the pressure in the front chamber, one only has to make a hole in the front chamber and place a pressure sensor opposite such hole. As for the measurement of the pressure in the rear chamber, the choice taken consists in drilling the partition wall between the chambers and in installing, in the opening thus made, a tight tube, extending within the front chamber and opening into a second hole, made in the latter. A second pressure sensor is set opposite this second hole. The tube is a flexible one and, furthermore, it enables the partition wall to move inside both chambers. Owing to the fact that the partition wall can take forward and backward positions, with a great range in the first chamber, the tube is manufactured in the shape of an unfolding spiral. The spiral pitch can be reduced or widened, depending on whether the partition wall is in a forward or a backward position in the chambers.
Because of the cost and, more particularly, on account of a material saving, the thicknesses of the chamber walls have to be reduced. Well then, it becomes evident that, owing to the motions of the partition wall and of the flexible tube, a pressure sampler head, for the pressure in the rear chamber, often in the shape of an elbow pipe, is subjected to stresses tending to break it or at least to separate it from the wall of the front chamber, to which it is fastened. More particularly, the tube exerts, on this head, bending and tilting forces which result in a possible loss of tightness at the attachment of said head to the wall of the front chamber.
Besides, the fact that there are two sensors gives rise to a second problem. The cost of such an implementation, i.e. the manufacturing cost plus the installation cost, heavily penalizes this solution from the economic point of view.
It is therefore an object of this invention to cope with the above-mentioned problems and to provide a pressure sampler head, which is designed so as to reduce, to an appreciable extent, the hyperstatic properties of the attachment, which constitutes a barrier without any tolerances to the forces exerted by the flexible tube during the travel of the partition wall. Thus in the invention, the tightness function and the mechanical holding function are separated as regards the pressure sampler head, by means of a detector baseplate rigidly mounted on the wall of the front chamber. The pressure sampler head comprises a sample tube which is maintained, in a fluidproof manner, at its lower part on the baseplate. Besides, this lower part is mechanically kept in contact with the baseplate through a collar bearing on the shoulder-forming edge of an opening made in a yoke-forming cage. In such a way, the tube may be slightly moved in the opening, both sideways and in the depth direction. That is to say that, though the sample tube should not be considered to be mobile, it is capable of slight motions and, this being so, whatever its position may be during such motions, the tightness of the connection is kept.
Of course, this solution will be preferred for the implementation of a double detector. As a matter of fact, the cage is perforated, either at its side wall or at its base, so as to communicate with the inside of the front chamber. Preferably, one hole only is made in said front chamber and the sample tube passes through this hole, with a sufficient clearance to define two spaces at the hole: one, inside the sample tube (connected besides by the flexible tube to the rear chamber), one free space about the ample tube and connected to the front chamber via a hole in the cage. Both passages are preferably concentric.
It results in that the sample tube is mechanically retained, that it can tolerate some stresses thanks to slight motions, and in that it ensures the tightness, by means of a seal which is always fluidproof, whatever the position of the tube may be in the course of its motions.
Therefore, it is an object of the present invention to provide a pneumatic servobrake, comprising a front chamber, capable of being connected to a vacuum source, a rear chamber, which can be connected to a high pressure supply inlet, a moving tight partition wall between said chambers, a moving set carried along by the partition wall and connected to a hydraulic braking circuit, a device provided for the supply of the rear chamber with a high-pressure fluid at the time of a braking operation, an opening in a wall of the front chamber, and a detector opposite such opening for the measurement of the pressure in at least one of the chambers, characterised in that said detector comprises a baseplate and a sample tube, the baseplate being connected, by means of a single rigid link, to the wall of the front chamber, whereas the sample tube is fastened, in a fluidproof manner, by its lower part to the baseplate, such tube comprising a collar, and a yoke-forming cage being fastened to the wall of the front chamber, the collar resting on a shoulder-forming edge of an opening made in the cage.
This invention also deals with an assembling process for a pneumatic servobrake, wherein:
a front chamber is provided, which is capable of being connected to a vacuum source;
a sample tube is inserted through an opening made in a yoke-shaped cage, a collar of the sample tube resting on a shoulder-forming edge of the opening;
the cage is fastened to an inner wall of the front chamber, opposite a hole in the front chamber, said wall having such hole opposite said opening;
a joint for the front chamber is inserted into the hole, so as to fit around the sample tube, and then an adapter of a baseplate of a one-piece detector, intended for the measurement of pressures, is inserted into said joint.